Huge shock absorbers, walls that slide and Teflon foundation pads that isolate buildings from the ground all help explain why medium- and high-rise structures in Japan remained standing in the wake of the country's largest earthquake on record, construction experts said Friday.

Since the devastating Kobe temblor in 1995, Japan has become a world leader in engineering new structures and retrofitting old ones to withstand violent shaking.

"The Japanese are at the forefront of seismic technology," said Eduardo Kausel, a professor of civil and environmental engineering at MIT. "All modern structures have been designed for earthquakes."

Strong Japanese building codes specify rules for short, medium and tall buildings, said Ron Hamburger, senior principal at the engineering firm Simpson Gumpertz and Heger in San Francisco.

New buildings shorter than three stories are required to have reinforced walls and foundation slabs of a certain thickness, meaning "there is not a whole lot of design to it," Hamburger said.

Mid-rise buildings, those up to 100 feet, require much more-intensive engineering, while designs for high-rise structures often employ innovative earthquake-resistant designs that undergo rigorous review by the country's top structural engineers.

Other shorter and mid-rise buildings rest on Teflon-coated pegs embedded in the foundation. The weight of the structure anchors the building on the pegs, but when the ground shifts the entire building slides over the smooth surfaces.

This technique is one of many "base isolation" methods that decouple buildings from the ground beneath, rendering them subtly floating structures.

In the United States, such techniques grew in prevalence after the 1989 quake that hit the San Francisco Bay Area. The historic, mid-rise city halls of San Francisco, Oakland and Los Angeles were all retrofitted to rest on giant shock absorbers.

Taller buildings employ more sophisticated measures, often in combination.

All modern skyscrapers are engineered to be strong yet flexible, so they sway in the wind - a discomfiting sensation felt on the observation decks of super-tall buildings such as the Willis Tower (formerly the Sears Tower) in Chicago.

Designing extra flexibility into the tallest buildings is essential for earthquake-proofing, said John W. van de Lindt, a civil engineering professor at the University of Alabama.

Video filmed during the aftershocks that hit Tokyo on Friday shows high-rises doing exactly that - wavering dramatically without snapping.

"You will get shelves tipping over and copy machines running across the floor," said van de Lindt, but structural damage will be minimal, even when the top of the building lurches 10 feet or more in each direction. "It's like a yardstick when you bend it - it snaps back without any damage."

Hollow walls hiding sliding metal plates are also common in recently built mid- and high-rise buildings in Japan. The heavy plates help dissipate motion.

The most sophisticated systems employ fluid-filled shock absorbers that slosh thick oil in the opposite direction of any swaying. One of the tallest buildings in Tokyo, the 781-foot Roppongi Hills Mori Tower, included such a "semi-active oil damper" design when completed in 2003.

Although the high-rises of Japan may be in good shape, experts worry that traditionally built houses fared much worse.

"My strong feeling is that there are collapsed wooden buildings in the hills and rural areas over there that we don't know about yet," said van de Lindt, who has tested V-shaped braces and other relatively simple techniques for strengthening wood structures in Japan.

Some 2,200 people died in wood-and-tile homes during the Kobe earthquake, van de Lindt said. That toll prompted the Japanese government to launch an intensive research and retrofitting program - called "Dai-Dai-Toku," or, roughly translated, "very, very special" - to prevent a similar catastrophe.

It remains unclear how those efforts performed Friday, but van de Lindt and other National Science Foundation-funded researchers will find out soon. The agency is sending a dozen or more rapid-response teams to Japan to evaluate the damage and gather information from damaged and collapsed structures.

"The idea is to get perishable data," which will then be used to further improve earthquake-resistant construction, van de Lindt said.